It's basically a "Monty Python" movie. Graham Chapman, John Cleese, and Eric Idle--arguably the front-liners of the Monty Python troupe--all have roles in the film. John Cleese reportedly didn't like the script, but was in the film as a favor to Chapman.
The title character, played by Graham Chapman (of "Monty Python" fame) is the utter worst of pirates. "Often forcing his victims to eat their own lips...," the opening exposition says.
The movie is full of humor. There are things in it which don't make a lot of sense, though. The casting of Cheech (Marin) and (Tommy) Chong, as Spanish priests, seemed odd, and Chong's feigned speech impediment in his role as El Nebuloso was distracting at best, and was never even remotely funny. In fact, the segments with Cheech and Chong are easily the weakest parts of the movie. The humor of that duo was built around recreational drugs, and their chemistry didn't work well in a pirate movie.
The movie centers around Yellowbeard's treasure. The pirate has been imprisoned and tortured in the hopes that he will reveal the location of his horde; whoever finds it will be wealthy beyond dreams of avarice. But Yellowbeard is made of sterner stuff and withstands twenty years of horrible conditions. On the day he's to be released, Eric Idle (as Commander Clement) informs him that he'll be serving another twenty years because he hasn't died yet. Yellowbeard breaks out of prison and goes to the home of Betty (played by Madeline Khan).
Betty is the last woman Yellowbeard raped before being arrested. She somehow ended up with his treasure map, and the son which resulted from their union--Dan--got that map tattooed on the back of his head.
Dan now works as a gardener at Queen Anne's castle, under the tutelage of Lord Lambourn, played by Peter Cook.
Betty visits Yellowbeard on the day of his release (or not, as the case may be) and tries to tell Yellowbeard about the existence of his son. She tries to approach the matter indirectly:
Betty: I'm talking about the fruit of your loins.
Yellowbeard: What are you talking about, woman? I haven't got fruit in my loins! Got crabs, though, and I'm proud of 'em!
Yellowbeard is not very intelligent, and somehow "fruit" and "crabs" become commingled into "prawn" (shrimp) and this exchange takes place later:
Yellowbeard: What, been out rapin', lad? Nice work, lad.
Dan: No, I haven't raped her.
Yellowbeard: No, you wouldn't have, would you, you poncy little git? You're not the prawn of my loins; your mother's a bloody liar!
The good thing about the movie is that it has many of these moments in which the viewer is expected to see the humor without the movie calling much attention to it. Overall the humor in the movie works very well while the scenes concentrate on the British cast.
It hits a brick wall whenever Cheech and Chong hit the screen. Ideally the roles played by those men should have been played by Python actors; Michael Palin would have been a natural for the role of El Nebuloso, and the role of El Segundo could easily have been pulled off by Terry Jones--and to better effect, at that.
This is not to say that Cheech and Chong are not funny--far from it. But as I said above, their humor is not well-suited to this kind of movie. Their chemistry works best in a modern framework; casting them as priests on the Spanish Main did not make much sense. Although they brought a lot of comedic horsepower to the film, there was no way they could make use of it given the plot and setting. Relegated to the "B" story, they were just tedious.
The movie suffered from having too many stars in it; and each star had to get his moment in the spotlight, so each and every one of them had his own sequence--this made many of the movie's "acts" too long, slowing the pace of the film to a tedious crawl in spots. Whenever Yellowbeard is ("and/or Dan are"?) on the screen, the movie works perfectly; when the action switches to others, however, it slows down or becomes uninsteresting. Even though Commander Clement is the primary antagonist, the movie spends too much time following his actions.
Even so, that would have been fine if the movie had not also spent almost as much time following the pirates Gilbert (Marty Feldman) and Moon (Peter Boyle) in their quest for Yellowbeard's treasure. And the worst part about the latter elements is that I can't even recall how that conflict was resolved, or even it it was resolved at all. Although the inclusion of these characters made for some funny scenes partway through the movie, it may not have been worth the drastic reduction in the speed of pacing. The funniest scenes would have been just as funny if they had been played by relative nobodies, and the writers would not have needed to include so much extraneous material.
But overall the movie is still worth renting from the bargin bin at the video store; it's got some wonderful chemistry and enough one-two punches to make it worth watching.